Much research has been performed during the last years on using multiple transmit and receive antennas (MIMO) for delivering high data rates over wireless channels. Different multi-antenna methods exploit the different properties of radio channels in order to leverage one or more of the array-, diversity-, and spatial-multiplexing gains. Spatial multiplexing, for instance, may increase the peak data rates and the spectral efficiency of a multi-antenna system. Several well-known transmitter and receiver architectures have been proposed to extract the promised multiplexing gains. One example is the Vertical Bell Labs Space Time Architecture (V-BLAST). Furthermore, a horizontal MIMO structure in which each one of the encoded streams are modulated and transmitted over different transmit antennas, known as Per Antenna Rate Control (PARC), attracts more and more attentions due to good performance. In PARC, the coding rate and the modulation of the stream transmitted from each antenna is controlled based on channel quality information that is, e.g., sent to the transmitter by the receiver. Depending on channel conditions, the transmitter might decide to use a subset of the transmit antennas only. This scheme is known as Selective Per-Antenna Rate Control (S-PARC).
Hybrid ARQ (HARQ) is a way to achieve reliable data delivery in a data communication system. HARQ allows combining features of a pure Forward Error Control (FEC) scheme and a pure Automatic Repeat reQuest (ARQ) scheme. Error correction and error detection functions are performed along with ACK/NACK feedback signaling. HARQ techniques have been adopted by several wireless standardization bodies, for example 3GPP and 3GPP2. HARQ can improve throughput performance, compensate for link adaptation errors, and provide a finer granularity in the rates effectively pushed through the channel. Upon detecting a transmission failure, e.g. by cyclic redundancy check (CRC), the receiver sends a request to the transmitter for retransmission.
Several efforts have focused on HARQ transmission in MIMO systems, e.g. for so called MIMO multiple ARQ (MMRQ) providing an efficient combination of MIMO and HARQ structure that yields more than 30% gain in link throughput in a MIMO system using per-antenna encoders (more specifically to have one ARQ process per stream) as described, e.g., in the document “Multiple ARQ processes for MIMO systems” by H. Zheng, A. Lozano, M. Haleem published in EURASIP Journal on Applied Signal Processing, 2004.05, p. 772-782.
HARQ transmission schemes in MIMO systems include, e.g., MIMO single ARQ (MSARQ) and MIMO multiple ARQ (MMARQ). With MSARQ, HARQ simply attaches a single CRC to the radio packet with a CRC encompassing the data radiated from the various transmit antennas. With MMARQ, multiple ARQ processes are employed in the MIMO channel, i.e. a CRC symbol is appended to each sub-stream. These MIMO-HARQ schemes, however, provide an indiscriminate service for all type of radio packets, i.e. they do not consider the characteristic of the radio packets. This is disadvantageous as usually different types of radio packets can have different priorities to transmit on the shared transmission channel. In addition, the first transmission and each retransmission attempt in HARQ might experience different channel qualities. To reach a good diversity gain in HARQ, either IR or CC, the transmission decision should be optimized or adaptive to the instant channel quality. Here the transmission decision includes both the antenna selection and the stream-wise selection for each transmission attempt.